One of the factors determining the effective range of a radar system is the polarization of the radiated electromagnetic waves. Radiation of vertically or horizontally polarized electromagnetic waves during normal weather conditions gives good target returns. However, the individual droplets in a dense fog or in rainy weather act as targets and reflect the electromagnetic energy so that the range of the system is substantially reduced. Further, the returns from the droplets make it difficult to distinguish rain returns from a true target. To overcome that difficulty, the radiated electromagnetic wave can be polarized so that the returns from symmetrical targets can be differentiated from the returns from non-symmetrical targets. This is accomplished by radiating circularly polarized wave energy. By radiating circularly polarized energy, the returns from symmetrical targets can be made to cancel whereby the returns from asymmetrical targets are readily observed. When circular polarization is employed, the returns from asymmetrical targets are generally weaker than returns from plane polarized waves but the elimination of much of the interference from rain and fog has proven the value of circular polarization in radar, and especially in those applications where maximum range is not an important factor.
There are a number of methods for causing electromagnetic energy to be circularly polarized. The device that causes vertically or horizontally polarized waves to be converted into circularly polarized waves is known as the "polarizer". A conventional polarizer may, for example, employ a dielectric slab disposed diagonally across a square or circular section of waveguide. The dielectric slab polarizer must be of such length as to cause a differential phase shift of 90.degree. in one component of the electromagnetic wave relative to the other component. Another type of polarizer uses a tapered fin polarizer such as is shown in U.S. Pat. No. 3,500,460. In that type of polarizer, the tapered fin is centrally situated in a hollow square waveguide and must be of such length as to cause a 90.degree. differential phase shift in the guide. Both of the foregoing types of polarizers require the 90.degree. phase shift to be accomplished within the waveguide and therefore the polarizer cannot be shorter than the waveguide length necessary to obtain the requisite 90.degree. differential phase shift.
Radiation of circularly polarized wave energy into space is usually accomplished in radar applications by means of a horn. For efficient transmission, the horn must be matched to the output of the polarizer. Thus, an impedance matching section may be required between the output of the polarizer and the input to the horn. In any event, the usual circularly polarized wave energy launcher is at least as long as the combined lengths of the polarizer and the horn and will usually be longer to accommodate matching or transforming structures.
In many applications, it is desirable to employ a circularly polarized wave energy launcher that is as compact as possible. For example, where the device is to be employed in an automobile as a collision alarm radar, available space in the vehicle may necessitate a compact wave launcher. In other applications, such as police radar, it is desirable to have the radar set as unobtrusive as possible. Furthermore, a consideration reduction in the size of a wave energy launcher makes small hand carried radar sets realizable with the present state of the electronics art.